1. Field of the Invention
The present invention relates to a light receiving circuit for detecting a change in amount of light, and more particularly, to a light receiving circuit capable of stable detection of a change in amount of light regardless of ambient light conditions.
2. Description of the Related Art
Light receiving circuits are used for receiving optical signals in infrared remote control communications or visible light communications and used for light-reflection type distance sensors using photointerrupters or trigonometry. The light receiving circuit absolutely needs to function to detect a change in amount of light accompanying ON/OFF of light or a change in amount of incident light or reflected light, but also needs to be capable of detecting the change in amount of light regardless of ambient light conditions.
FIG. 11 illustrates a conventional light receiving circuit. The conventional light receiving circuit is constituted by a photodiode 101, a resistive element 150, a low pass filter 501, an N-channel MOS transistor (hereinafter, abbreviated to NMOS transistor) 102, and an output terminal 104. The photodiode 101 has a photoelectric conversion function for converting an optical signal or a change in amount of light into a current change. The resistive element 150 converts the current change obtained by the photoelectric conversion of the photodiode 101 into a voltage change. The NMOS transistor 102 has a gate supplied with a drain voltage via the low pass filter 501. The output terminal 104 outputs a change in voltage generated across the resistive element 150.
The photodiode 101 has an N terminal connected to a VDD terminal, and a P terminal connected to the output terminal 104, one terminal of the resistive element 150, a drain of the NMOS transistor 102, and an input terminal 505 of the low pass filter 501. The other terminal of the resistive element 150 is connected to a GND terminal. The low pass filter 501 has an output terminal 506 connected to the gate of the NMOS transistor 102. The NMOS transistor 102 has a source connected to the GND terminal. Although not illustrated, the VDD terminal is supplied with a positive voltage from a power source and the GND terminal is supplied with a reference voltage from the power source.
The light receiving circuit having the above-mentioned configuration operates as follows to detect a change in amount of incident light.
When the environment is dark, no steady current flows through the photodiode 101, and hence a voltage at the output terminal 104 is almost a GND terminal voltage and the NMOS transistor 102 is OFF. Therefore, a voltage to be output from the output terminal 104 is a voltage generated when the current of the photodiode 101, which changes as the amount of light entering the photodiode 101 changes, flows through the resistive element 150. On the other hand, when the environment is bright, a steady current flows through the photodiode 101, and hence a potential difference across the resistive element 150 is increased by the current. If the flow of the steady current through the photodiode 101 increases the potential difference across the resistive element 150 to exceed a threshold voltage of the NMOS transistor 102, the output terminal 104 is controlled by the NMOS transistor 102 so as to be around the threshold voltage of the NMOS transistor 102. In other words, however bright the environment is, the voltage of the output terminal 104 is not increased to a VDD terminal voltage but increased to around the threshold voltage of the NMOS transistor 102. Therefore, the output terminal 104 outputs a voltage waveform that has no peak at the VDD terminal voltage, with the result that even if the environment is very bright, the output voltage is changed by the change in amount of light. In other words, the change in amount of light can be detected regardless of ambient light conditions.
When the environment is bright and the voltage at the output terminal 104 is controlled to be around the threshold voltage of the NMOS transistor 102, a current flows through the NMOS transistor 102, too. However, a gate voltage of the NMOS transistor 102 is changed via the low pass filter 501, and hence the change rate is slow. Besides, the low pass filter 501 is set so as to pass only a signal at an extremely low frequency. Therefore, the current flowing through the NMOS transistor 102 is regarded as a constant current with respect to an instantaneous current change, and hence the NMOS transistor 102 has little influence on lowering the light receiving sensitivity.
In addition, in order not to detect a slow change in amount of light that occurs by a person moving across the sensor, a human hand coming close thereto, a curtain waving in the wind, or the like, the low pass filter 501 is set to have a pass frequency capable of passing frequency components of a voltage change accompanying a current change that occurs by the change in the amount of light.
Further, although not illustrated, the conventional light receiving circuit needs, at the output thereof, an input circuit for converting a signal of less than CMOS level, which is to be output from the output terminal of the conventional light receiving circuit, into a signal of the CMOS level.
In the conventional light receiving circuit described above, the current of the photodiode is allowed to flow through the NMOS transistor in which the source is connected to the GND terminal and the drain voltage is supplied as the gate voltage via the low pass filter. Accordingly, even if the environment is bright and the current of the photodiode 101 is large, the output voltage is prevented from exceeding around the threshold voltage of the NMOS transistor 102. Therefore, the output voltage is changed by the change in amount of light regardless of ambient light conditions (see, for example, Japanese Patent Application Laid-open No. Hei 09-083452).
As described above, the conventional light receiving circuit has a configuration in which the output voltage varies based on the change in amount of light regardless of ambient light conditions. However, as described above, when the environment is dark, the conventional light receiving circuit outputs a voltage that changes mainly around at the GND terminal voltage according to the change in amount of light. When the environment is bright, on the other hand, the conventional light receiving circuit outputs a voltage that changes mainly around at the threshold voltage of the built-in NMOS transistor according to the change in amount of light. Accordingly, if the output of the conventional light receiving circuit is input to a simple CMOS level input circuit such as a CMOS inverter, when the environment is dark, the input signal cannot be detected because a level of the input signal is low and a voltage change accompanying the change in amount of light is very small. On the other hand, when the environment is bright, a through current flows to the CMOS level input circuit because the input signal is not at the CMOS level or more. Therefore, the input circuit at the subsequent stage needs to use an amplifier circuit or the like so as to have a wide DC voltage range with respect to the input voltage. In this case, however, the input circuit at the subsequent stage has a complicated, expensive configuration, and the use of an amplifier or the like steadily consumes a current by the amplifier circuit. In other words, the conventional light receiving circuit has a problem that an input part at a subsequent stage is complicated and expensive and also a consumption current of the input part at the subsequent stage is increased. In addition, all the current of the photodiode flows to the GND terminal, and hence there is another problem that when the environment is bright, a consumption current of the light receiving circuit is increased. Besides, in order to allow the conventional light receiving circuit to detect a weak change in amount of light, such as an optical signal output from a remote place, the light receiving sensitivity needs to be increased. However, in order to increase the light receiving sensitivity of the conventional light receiving circuit, the above-mentioned resistive element needs to be increased in resistance. In other words, the conventional light receiving circuit requires high resistance of the above-mentioned resistive element for increasing the light receiving sensitivity, with the result that the occupied area of the above-mentioned resistive element becomes too large to form the light receiving circuit within an IC chip. Therefore, there is another problem that it is necessary to add an external resistive element, which is expensive and requires a space.